The idea of using background that is recognizable is pretty good one.
However, T. Pornin has been kind enough to point out some issues with that approach.
Short: Use MobiCore or other TrustZone implementations where available. For untrusted devices without TEE, currently you need to use Rich OS only. In future, maybe more devices offer TEE.
Issues with trust and confirmation
Ultimately, the issue is: if you cannot trust components and software executing on the device you cannot trust the device to not change the confirmation message/button shown to the user.
There is no perfect solution for this problem (asking confirmation on untrusted device). This is a widely known fact. There are even occasionally issues with devices generally regarded as "trusted" (e.g. ATM). So, banking and credit business is always expecting some amount of fraud and prepared to deal with it.
For many applications, currently it is regarded to be secure enough having applications running directly on untrusted mobile devices. (Such as online banking etc.)
Trusted Execution Environment
What if it was possible to have trusted environment for use cases needing extra trust? (For use cases mobile commerce, BYOD, etc.)
Network operators and hardware manufacturers formalized much of requirements for Trusted Execution Environment useful for running "trusted software" in trusted environment on device which non-trusted OS (also known as HLOS, Rich OS, General Purpose OS, or Normal World OS). These documents are OMTP TR0 and OMTP TR1.
They are one of best presentations of usual requirements for TEE.
ARM+Trusted Logic started working on this idea around ten years ago. ARM processors got TrustZone, which has been there since. Although largely good idea in practice, big issue was: there is support on hardware, but software solutions did get it utilized all that much, until recently.
Technically, it is possible to consider TrustZone as limited HW support for virtualization, with intention to use one rich OS and one secure OS.
G&D MobiCore has been one of few implementations that have gotten to major mobile phones, it is available on Samsung S3, S4. Samsung Note III has Samsung Knox. I think it also is based on MobiCore on TrustZone.
Some other phones have some other software. In many cases, there are no details available of what phones do with software they could or do load on TrustZone, which is available on almost all current smart phones.
To get proper SW support for TrustZone, G&D, ARM and Trusted Logic Mobility together founded Trustonic, which is making the next generation Trusted OS